The present invention is related to lamps, particularly sealed beam lamp units, having two-piece envelopes comprising a reflector and lens assembled by adhesive means. The reflector has an internal reflective coating for reflecting and directing light, originating from a light source located within the envelope, towards a cooperating lens through which the light is transmitted. Such sealed beam lamps have particular utility and are commonly used as headlights for motor vehicles.
Lamp units, such as headlights, recently have been introduced with lenses and reflectors having rectangular shaped peripheries, supplanting the more familiar circular units. Production of assembled rectangular glass reflectors and lenses, however, can present numerous problems. For example, stresses created in the glass lenses and reflectors during assembly by fusion sealing can cause cracking thereof. Such stresses can be significantly reduced by using an adhesive, rather than fusion, to seal the glass reflector and lens together. However, the ambient temperature, particularly at cold temperatures, can produce additional stresses resulting in cracks especially about the reflector corners. Thus, the inherent problem of thermally induced stresses experienced when dissimilar materials, such as glass and adhesive, are joined remains.
More particularly, typical coefficients of thermal expansion for glass, such as borosilicate, and of a suitable adhesive for bonding lamp glassware, such as an epoxy, which has been flexibilized by the incorporation of a polymer, can differ by a factor of about 10. Thus, the glass-adhesive seal when exposed to a decreasing ambient temperature can have glass portions thereof contracting at a much different rate than the adhesive portions thereof. Such variations in contraction create stresses, especially around the reflector corners, resulting in spalling of the adhesive to glass interface and cracking of the glass.
Additionally, if any portion of the adhesive flows onto the light-transmitting area of the lens or light-reflecting area of the reflector, or beyond the lamp unit periphery during assembly of the unit, undesirable and unacceptable lamp unit optical performance and/or peripheral dimensions can result.